1. Field of the Invention
The optical data transmission capacity of systems using wavelength multiplexing can be increased by the use of integrated optoelectronic circuits. The range of usable wavelengths is usually limited, however, for at least two reasons: the need to distribute the wavelengths between the various types of system application (direct or coherent detection, local area networks or long-haul transmission, etc) and the possible benefits of working within the bandwidth of available optical amplifiers (rare earth doped optical fiber amplifiers, semiconductor amplifiers, etc).
2. Description of the Prior Art
The usual problem with increasing the capacity of transmission systems using wavelength multiplexing is to insert a maximal number of channels within a given bandwidth. This involves reducing the spacing between channels at the sending (multiplexing) end and at the receiving (demultiplexing) end.
One prior art multiplexer comprises a dispersive group constituting a diffraction grating and is in the form of a wafer comprising a guide layer disposed between two confinement layers, said guide layer comprising guide parts having a refractive index higher than that of said confinement layers whereby it is adapted to guide in its own plane optical waves constituting waves internal to said multiplexer, which comprises a multiplexing channel having:
a guide part constituting a two-dimensional guide area delimited by an edge to enable said internal waves to propagate in various directions in said plane away from and towards said edge; PA1 a linear output guide having an entry at a point on said edge to receive internal waves from said guide area and to guide them in the direction of said output guide, said output guide connecting said entry to an output of the multiplexer to convey thereto a composite wave produced by superposing the internal waves that it receives; PA1 a limited section of said edge having a dispersive group formed by a periodic succession of points diffracting light to constitute, for internal waves propagating towards said section, a diffraction grating adapted to divert said waves into said guide area in directions dependent on their wavelength, said grating being concave to focus said waves; and PA1 a plurality of light injectors carried by said wafer for injecting respective internal waves into said guide area towards said diffraction grating from an injection point on said edge so that said grating focuses said wave towards said entry of said output guide and injects said wave into said output guide, said wavelengths of said internal waves being staggered in corresponding relationship to said plurality of injectors, and said injection points being staggered in corresponding relationship to said wavelengths to achieve said focusing of each internal wave at said output guide entry.
These diffraction gratings are called Fresnel mirrors. In the prior art the injection points, the diffraction grating and the output guide entry are preferably on a common "Rowland" circle.
To increase the number of channels grouped together by a multiplexer of this kind without increasing the overall bandwidth entails reducing the spacing between adjacent channels without causing crosstalk between the channels, and to achieve this it would seem necessary to increase the resolution of the diffraction grating employed.
Various solutions to this problem have been proposed. One is to increase the focal length of the diffraction grating. It requires the size of the wafer constituting the multiplexer to be increased in the same proportions, which is a major drawback in the field of telecommunications.
Another is to reduce the pitch of the grating. This runs up against a technological limit set by the methods available for etching the grating. This limit depends on the material of the wafer. It is higher for systems using GaInAsP materials than for the Si.sub.2 /Si system. The latter has advantages in terms of cost and ease of integration, however. Whatever material is used it is necessary to increase the resolution of the multiplexer, i.e. to reduce the gap that has to be maintained between adjacent channels to prevent crosstalk.
One object of the present invention is to increase the number of channels in a given bandwidth which can be grouped by a multiplexer of limited size without causing crosstalk.